This invention relates generally to semiconductor manufacturing systems, more particularly to cleaning apparatus forming a part of semiconductor manufacturing systems, and most particularly to high-purity cleaning systems for flat workpieces.
The production of integrated circuits requires very clean systems and processes. This is because integrated circuits and other semiconductor devices often have line-widths in the submicron range. Even very tiny particles adhering to the surface of a semiconductor wafer during the integrated circuit manufacturing process have the potential of destroying the functionality of integrated circuits on the wafer. The semiconductor manufacturing industry therefore goes to great lengths to keep the semiconductor manufacturing equipment, and the surrounding environment, in a very clean condition.
Semiconductor manufacturing is typically accomplished within xe2x80x9cclean roomsxe2x80x9d which are categorized by xe2x80x9cClass.xe2x80x9d For example, a Class 100 clean room will have no more than 100 airborne particles of a certain size per cubic foot. A Class 10 clean room will have only 10 such particles per cubic foot, a Class 1 clean room will have only 1 particle per cubic foot, etc. In modern semiconductor manufacturing plants, Class 1 clean rooms are often used.
Integrated circuits are typically formed in multiple copies on a single semiconductor wafer. This semiconductor wafer used most frequently is made from a very pure silicon material; and are typically 6-12 inches in diameter. These wafers, as received from the manufacturer, are very clean. They may, for example, have fewer than a dozen particles of a given sub-micron size per wafer. Ultimately, these wafers are processed by a series of deposition, masking, etching, implantation, etc. operations in order to form a number of integrated circuits on a surface. The wafer is then cut into xe2x80x9cdiexe2x80x9d, each of which includes a single integrated circuit chip. Operative die are packaged and sold as the final integrated circuits.
The xe2x80x9cyieldxe2x80x9d from a semiconductor wafer is defined as the total number of operative die divided by the total number of die on the wafer. The cost-per-integrated circuit is directly related to the yield of the wafer. Since a leading cause of inoperative die are particles that were present on the surface of the wafer during the manufacturing process, it is imperative to keep the wafer surface as clean as possible during the manufacturing process.
Unfortunately, many of the processes to which a wafer is exposed during the manufacturing process are inherently dirty. For example, an etching process can create a large number of particles on the surface of a wafer. As another example, chemi-mechanical polishing (CMP), which is increasingly used to replace isotropic etching processes, uses chemicals and fine particles to grind the surface of a wafer. Chemi-mechanical polishing is also sometimes referred to as xe2x80x9cchemi-mechanical planarizationxe2x80x9d, since it is often used for planarization purposes. It-will therefore be appreciated that CMP processes generate a great many particles on the surface of the wafer which must be removed to maintain a reasonable yield from the wafer.
The prior art teaches a number of methods for cleaning wafers. The most common is an aqueous or chemical bath into which the wafer is dipped. However, these xe2x80x9cwetxe2x80x9d or xe2x80x9cdipxe2x80x9d processes have a number of deficiencies. For one, wet processes are generally incompatible with cluster tool processing. Wet cleaning processes have traditionally been batch processes where a group or batch (usually 25) wafers was cleaned, rinsed, and dried together in a cassette. Since cluster tools process wafers singly (in series), wafers from cluster tools must be loaded into cassettes and then transported to cleaning areas by human operators. This requires that the wafers be loaded into plastic cassettes that are placed into dip tanks for chemical processing. These cassettes are then placed into spin/rinse dryers for final rinsing and drying. This process consumes time, costs money, and exposes the wafers to the possibility of contamination during transport. As wafer sizes get larger, the transportation and handling of wafers in cassette batches becomes much more difficult and less practical.
To aid in the removal of particles, traditional wet cleaning equipment has used ultrasonic (or Megasonic) high frequency agitation, rotating brushes, or high velocity liquid jets. High frequency agitation has been proven effective at removing particles, but the difficulty lies in preventing redeposit of particles from the solution once the ultrasound energy stops. Since the wafer must often be removed from the solution through a gas liquid interface, it must be removed through a zone where particles may concentrate, recontaminating the wafer upon exit. To avoid this, it has been proposed to provide continuous liquid flow at the interface, but high overflow rates are required to keep particle counts low and this may result in excessive water or chemical consumption.
Rotating brushes have also been used, where the bristles have been chemically treated to modify solution Zeta potential, therefore attracting particles from the wafer surface to the brush. However, for these to work, the nature of the particles must be such that they are attracted to the charge on the brushes. In addition, cleaning the brushes may be critical since dirty brushes (having accumulated a lot of particles) will eventually recontaminate the wafer surfaces, reducing cleaning effectiveness. Liquid jets must be very high velocity in order to result in a fluid boundary layer on the order of the particle sizes (well below 0.5 micron). These high velocities can damage surfaces due to erosion, especially with patterned substrates with various surface topography.
Wet processes do not tend to be very effective at removing all particles, and will actually add particles to the surface of the wafer when the cleaning solution becomes dirty. In addition, the aqueous or liquid phase contains particles that are about 3 orders of magnitude higher (per cubic meter) than those found in the gas phase. This is in part due to the fact that filtration technology is about two to three orders of magnitude less effective for the liquid phase than for the gas phase. Further, even the purest of water has the propensity to grow contaminating microorganisms. Because of these factors, there has been a dedicated focus for many years on xe2x80x9cdryxe2x80x9d processing utilizing gas phase processes.
The number of particles added to a wafer by the solution is dependent on the concentration equilibrium between the particles in the solution immediately in contact with the wafer and those on the surface. Soaps and surfactants will effectively reduce the xe2x80x9capparentxe2x80x9d particle concentration by tying or solvating the particles to organic components in solution. Soaps are not widely used in the semiconductor industry for cleaning of high purity Si wafers because these same surfactants will also contaminate the wafer surfaces. In the case of very dirty wafers and very clean solutions, there will be a tendency for particles to move into solution (assuming charge effects and solution chemistry permits this). In the case of clean wafers and dirty solutions, the opposite can occur. The ultimate baseline test of a cleaning system is to measure particles added to or removed from xe2x80x9cvirgin primexe2x80x9d substrates which have very few particles on their surface. The better the cleaning system, the fewer the particle adders will be seen.
A typical commercial wafer cleaning apparatus (such as a spin rinse dryer) will always add particles to prime substrates, even when using ultra pure deionized water. This is because, no matter how pure the water source, there is always present particle and bacterial contamination. Only the very sophisticated and highly proprietary final cleaning processes used by the original equipment manufacturers (OEMs) of the silicon substrates, i.e. the wafer manufacturers themselves, can actually remove particles from prime quality wafers. These processes are too complex and expensive to be used in the production fabs.
For the foregoing reasons, wet wafer process cleaning has always been deemed by production fabs as a necessary evil, and for many years effort was focused on developing so called dry cleaning processes which used reactive gasses and plasmas to try and remove particle contaminates. With the advent of CMP, the practicality of using dry processes to remove large levels of contamination has been considerably reduced.
The use of high purity steam has some potential advantages when compared to conventional wet cleaning. If done correctly, high purity water can be vaporized into a high purity gas (steam), then condensed directly onto the wafer surface. UHP steam is potentially devoid of any ions and contaminates, including bacteria and particles, and will be a very aggressive solvent for surface contaminants. However, it is very important that adequate amounts of steam are applied to the wafer surface, and that the contaminants are flushed uniformly from the surface. Condensing steam at 1 atmosphere pressure will also raise the wafer temperature to nearly 100xc2x0 C., making residue free drying a possibility.
The prior art has taught the possibility of using steam to clean silicon wafers for a number of years. For example, U.S. Pat. No. 4,186,032 and U.S. Pat. No. 4,079,522, mention the use of an inclined heat sink to process wafers one at a time by condensing steam on the wafer surface and allowing the condensate to drain off by gravity. However, this method will not produce uniformly clean wafers due to the fixed orientation of the wafer being cleaned, resulting in potential particle gradients bottom to top. Further, the process as described is quite lengthy, requiring many minutes per wafer. In addition, the technique has no provision for backside cleaning (i.e. the cleaning of the surface of the wafer opposite from the active surface of the wafer) that would not recontaminate the front surface. There is also no mention of purifying the steam prior to condensation, which is critical for particle free performance, since steam (like any gas) can contain aerosols and particles that will contaminate the wafer surface upon condensation.
The prior art discloses the use of steam or condensing water vapor to clean silicon wafers. Typically, such prior art discloses the contact of a wafer cassette or single wafer with saturated steam. The prior art therefore typically ignores the requirement to provide adequate xe2x80x9cheat sinkingxe2x80x9d behind the wafer to condense sufficient steam to clean. Without the heat sink, only enough steam will condense to raise the temperature of the wafer from its input temperature to 100xc2x0 C., e.g. only a few cubic cm. To condense sufficient quantities of water for adequate cleaning, on the order of a 100-200 cc/min of condensate, a heat sinking of many kilowatts is required. This is perhaps the reason condensing steam has not been utilized for conventional batch cassette processing. It is not easy or practical to heat sink each wafer adequately in a cassette.
It would therefore be desirable to have a cleaning method and apparatus which is both more effective than cleaning methods and apparatus of the prior art and which can become an integral part of a semiconductor manufacturing system.
One aspect of the present invention applies filtered, high purity steam to an active surface of a wafer to clean particles from the active surface. Preferably, a relatively cooler liquid is simultaneously applied to the back of the wafer to heat sink the large heat of vaporization and provide backside cleaning. Due to the elevated temperature produced by the condensing steam, the wafer can be dried quickly in situ. Because the invention does not use the xe2x80x9cwetxe2x80x9d batch (cassette) cleaning technology of the prior art, the clean wafer can exit from the system in a dry state, enabling a xe2x80x9cdry in/dry outxe2x80x9d single wafer processing strategy. Consequently, the cleaning apparatus of the present invention can be tightly integrated and form a part of a semiconductor processing system.
In one aspect of the present invention, a semiconductor processing system includes at least one semiconductor fabrication process apparatus operative to perform a semiconductor fabrication process on a surface of a semiconductor wafer, and a wafer cleaning device for cleaning the wafer surface preceding or subsequent to the semiconductor fabrication process. Such semiconductor fabrication processes include thermal oxidation, chemical vapor deposition, epitaxial deposition, physical vapor deposition, copper deposition, etch, or chemi-mechanical polishing. Preferably, the wafer cleaning device includes a wafer rotating mechanism, a steam inlet for applying steam directly to an active surface of a rotating wafer, and a liquid inlet for simultaneously applying a liquid to the backside of the wafer. In one embodiment, one or more wafer cleaning devices may be part of a cluster tool sharing a common transfer chamber and a common wafer transport arm with single or multiple semiconductor fabrication process apparatus. In another embodiment, one or more wafer cleaning devices may reside by themselves on a cluster tool separate from the cluster tool to which the semiconductor fabrication process apparatus are attached.
The present invention further includes a method for manufacturing an integrated circuit including subjecting an active surface of a wafer to a plurality of processes, and cleaning the active surface of the wafer before, during, or after the plurality of processes. As used herein, the term xe2x80x9cplurality of processesxe2x80x9d refers to two or more processes selected from a group including thermal oxidation, deposition, patterning, doping, planarization, etching, and ashing processes. For example, the planarization process can be a chemi-mechanical polishing (CMP) process. Cleaning the wafer further includes rotating the wafer, applying a vapor phase to the active surface at a first temperature, and applying a second liquid to the backside surface at a second temperature lower than the first temperature. The vapor phase may include steam, mixtures of steam and isopropyl alcohol, or mixtures of steam and hydrogen chloride, hydrogen fluoride, hydrogen bromide, and ammonia. The vapor phase may be filtered prior to application to the wafer surface. After subjecting the active surface to the plurality of front end semiconductor manufacturing processes, the wafer is cut into a plurality (i.e. two or more) of integrated circuit die, and the die are packaged to form a plurality of integrated circuits.
A work piece cleaning system in accordance with the present invention includes a work piece holder including a plurality of work piece gripping members, a rotator mechanism coupled to the gripping members to rotate the work piece, a vapor phase inlet positioned to apply a vapor phase at a first temperature to a first surface of the work piece, and a liquid phase inlet positioned, to apply a liquid phase at a second temperature lower than the first temperature to a second surface of the work piece. The liquid phase cools the work piece such that there is substantial condensation of the vapor phase when it contacts the first surface of the wafer. Preferably, the system further includes a liquid supply, and a vapor phase generator and filter coupling the vapor generator to the cleaning chamber. The liquid supply preferably comprises deionized water, and/or isopropyl alcohol solution, or other suitable aqueous and non-aqueous cleaning agents.
A work piece cleaning system in accordance with other aspects of the present invention includes a supply of an aqueous solution, a steam generator coupled to the supply of aqueous solution and operative to generate a vapor phase of the aqueous solution, a filter coupled to the steam generator to filter the vapor phase, a rotating wafer holding mechanism, and a nozzle coupled to the filter to direct the vapor phase to a surface of the wafer that is held and rotated by the wafer holding mechanism. Some of the vapor phase becomes liquefied upon contacting the wafer, and is urged outwardly toward the edge of the wafer by the rapid rotation of the wafer. Preferably, a liquid phase of an aqueous solution is applied to the other surface of the wafer to clean the backside of the wafer and to cool the wafer to aid in the condensation process. An ultrasonic transducer or other vibration mechanism can be coupled to the system to further aid in the cleaning process.
An advantage of the present invention is that it can be characterized as a xe2x80x9cdryxe2x80x9d process, even though various fluids are applied to the wafer during the cleaning process. This is because the wafer is heated during the cleaning process, and can be removed from the cleaning system in a dry state. As such, it can form an integral part of a semiconductor manufacturing apparatus, such as a cluster tool.
Another advantage of the invention is that the active side and the back side of the wafer can be cleaned simultaneously. This is advantageous in that processes of the prior art that only clean the active side of the wafer leave particles and residue on the back side of the wafer that can, itself, become a source of contamination for the active side.
A still further advantage of using a vapor phase, such as steam, to clean the active side of a wafer is that it is an inherently cleaner process than the wet clean processes of the prior art. This is due, in great part, to the extremely high purity that can be achieved with steam, especially when it is passed through a filter in its vapor phase. As such, the steam cleaning agent introduces virtually no particles or contaminants to the surface of the wafer.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions of the invention and a study of the several figures of the drawing.